Because of the advantages of reliability, low cost, and the potential for achieving high precision microsensors, several attemps have been made to integrate mechanical tuning elements on silicon substrates. A polysilicon microbridge has been driven vertically as a resonant microsensor; it features a simple process that ensures high yield, a high quality factor-Q-in vacuum, and high tunability, by varying physical or chemical parameters.
However, vertically driven microbridges have suffered from several shortcomings. At atmospheric pressure, the Q is severely reduced by viscous damping, usually to less than 20. Furthermore, it requires holding its driving signal to a very small value to sustain resonance in a vacuum, typically on the order of a few millivolts, thereby creating a serious hurdle in designing a stable sustaining amplifier. Because the microbridge is essentially a variable capacitor driven electrostatically, the relationship between the drive signal and the capacitance readout is highly nonlinear, presenting a challenging problem for the sense circuit.